Gravity meter



' Patented May 27, 1941 Gasvrrr mam Dayton H. Clewell, Dallas, Ten,assignor, by mesne assignments, to Socony-Vacuum Oil Company,Incorporated, New York, N. Y., a corporation of New York pplicationFebruary 21, 1940, Serial No. 320,05

3 Claims.

This invention relates to means for performing geophysical explorationsby measuring the relative gravitational forces which the earth exerts atvarious locations. More particularly, this fixed position at eachlocation and measuring (01. ass-1.4)

the amount of force necessary to bring the mass back'to that position:avoids the necessity of having to calculate the amount of force thatcauses a certain deflection of a complicated re-- invention relatesto aparticular means whereby 5 silient support for a gravity meter mass.Difiia resilientl supported mass may be brought to culty has beenencountered, however, in finding the same position relative to thesupport regarda way to apply force to a gravity meter mass less of smalldifferences in gravitational attracin such a way that the force can besmall enou h tion occasioned by changes in the location of the tobalance the gravitational diiference and yet instrument, and the forcerequired to bring the can be measured. Attempts to accomplish the massto the same relative position may be measresults electromagneticallyhave not been entireured as a measure of the difl'erence in gravitalysatisfactory partly because of the extremely tional attraction. smallforces that it is desired to applyhnd the Prior to this invention alarge number of gravfact that the current necessary to apply such a itymeters of various types have been proposed force must necessarily be sosmall as to make and many have gone into actual use. Most of measu eme te e y dlfli these have consisted of resiliently supported Confrontedwith this situation, the present in masses and arrangements for notingthe posiv n or ha discovered that by the useof the force tion of themass relative to the support s th t exertedelectrostatically betweenbodies at difany change in its position due to a change in the 2 fel'entelectrical potentials he may use a relagravitational force affecting itcould be noted. v y large voltage to apply to the gr y meter In ageneral way these devices may be considered mas-B an y small force.which force is to be refinements of the commonly used spring directlyrelated to themasnitude of t e vo ta e balance. Obviously, however, therefinements used, and which for that M88011 y fi have been highlycomplicated since it is necessary 2 m ed by mi ng the voltage' o p tomake the device far more sensitive and comdllce pensate for or excludeerrors due to tempera- This invention yb embodied in y gravity turechanges, fatigue in materials, stretching of meter of e im dy n n in h ha supporting wires, and th lik mass is suspended resiliently from asupport and In addition to these complicating factors, a means isprovided .to observe the position of which make accurate determinationof gravity the mass For example. from the s of such a diflicult, but notimpossible. it has als b gravity meter, a small ring of conductivematefound that when a mass is supported by an elasrial such as copper oraluminum may be suptic or resilient supporting means, and the mass p d,and above and bel0w t s r g, 0 011 P- then goes upward or downward, dueto a change posite sides of it in case it becomes desirable in thegravitational pull upon it, the reaction of place it in VerticalPosition, y be p d the resilient supporting means on the mass a D 01'fixed 8 P 0 conductive changes not in a single ratio to the movement ofma r l- W. by p g a potential betw n the mass but in a-complicated waythat is quite the two fixed plates and bringing the plate car-.difiicult to determine or compute. This is due, 40 ri fl by he gravitymeter mass to a potential at least in part, to the fact that thesupporting that may be varied between the potentials of he means isnearly always used under peculiar contwo fixed plates, a variable forcemay be exerted ditions to make it unusually sensitive to small n the m vp and t u'sh it on the ravchanges in gravitational force. Therefore,when ity meter ma s. The relative v ltage of. the a measurement is madeat one locality and a 4 able Plate and the fix d plates will govern thezero or base point established for the position m of f c mpli but theforce will be of the mass, and the instrument is subsequentm 1 ndproportional to the voltage. Thus, ly moved to another locality where itis found y measuring the pp ed voltage a measurement that the mass comesto rest in a different posimay be had of the force necessary to bringthe tion, it is usually almost impossible to ascertain gravity metermass back to a zero or poor compute the actual difference in thegravisition and this force is equal to the difierence tational forcewhich causes the displacement. in a. gravitational pull on the mass atthe loca- Attempts have already been made to avoid tiOnSin question; 7this difficulty by bringing the mass back to a In order to more fullyunderstand the details and advantages oi the present invention,reference may be had to the draWing in which Figure 1 is a diagrammaticillustration of the essential parts of a gravity meter embodying thepresent invention, Figure 2 is a detailed view showing the manner inwhich the movable ring sits in between thestationary ring and Figure 3is a view similar to Figure 1 but showing only a part of the deviceembodying a slightly different arrangement of the fixed or stationaryrings.

It will at once be apparent to those skilled in the art that manymodifications'of the structural details of the illustrated device may bemade without departing from the spirit of the invention. For example,any other desired type of support for the gravity meter mass may beused, a conductive disc may replace any one or all of the conductingrings shown, and in fact the gravity meter mass itself may in someinstances be used as the movable conductive memher. The device of thedrawing is shown merely for the purpose of illustrating. the principleshere involved and because it is one form of the invention that is knownto have particular advantages, but the scope of the invention is notlimited thereto.

The preferred embodiment of the invention shown in the drawing uses atype of gravity meter containing a support I, resilient torsion elements2 and a suspended mass 3. Inthe operation of this gravity meter 9.torsional stress is applied to the resilient elements in such a mannerthat the system is in a delicately balanced state and as a result thesuspended mass is very forces acting upon it. Any change in thegravitational force causes the suspended mass to be displaced in avertical direction and imparts to the mass a rotary screw action eitherup or down depending on whether the gravitational force increases ordecreases.

According to the invention the change in downward force exerted by thegravity meter mass is measured by applying a balancing force to the masswhich counteracts any change in the gravitational attraction and themagnitude of the force required to return the mass to or keep the massin its original position is an exact measure of the change in thegravitational force.

In the preferred embodiment this is accomplished by suspending from thecentral point of the gravity meter mass by means of a conductive element4, a. non-magnetic electro-conductive segment of a cylinder 5 in such amanner'that its axis is at right angles to the axis of the gravity metermass. Two fixed rings of conductive material Brand I are disposedconcentrically about the segment of the cylinder and spaced from thesurface ofthe cylindrical element 5 by small air gaps,.the ring 0 beingpositioned above and the ring 1 being positioned below the cylinder asshown in Figure 2.

A battery 8 is connectedacross the two rings 6 and 1. In the arrangementshown in Figure 1,

the positive pole of the batteryis connected to the upper ring 6 and thenegative pole to the lower ring I and results in the ring 6 beingcharged positively and ring "I being charged negatively. Thisapplication of potential across the two rings builds up an electrostaticfield befixed rings and may have a diameter of the same magnitude as theflzled rings. This embodiment is shown in Figure 3.

While the cylindrical element 5 is in the field of flux supplied by theelectrostatic field between the rings 8 and I, a potential is applied toit by means of a return circuit through the gravity meter mass 3 andsupport I and the suspension elements 2 and l. The gravity meter supportI is connected by a lead 9 to the battery 8 through a variableresistance H) which is connected across the battery. In this returncircuit is placed a voltmeter II to indicate the potential appliedthrough this circuit to the cylinder 5.

When a potential is applied to the cylinder, the flux of the fieldbetween the rings is changed and the cylinder is displaced in thedirection of the greatest flux. The displacement of the cylinder followsthe old theorem that like charges repel and unlike charges attract andthe amount of attraction or repulsion depends upon the strength of thecharges. In this case the strength of the charges on the two rings 6 and1 remains constant while the strength of the charge on the cylinder 5 isvaried by operation of the variable resistance II.

The movement of the cylinder 5 in the electrostatic field imparts acorresponding movement to the gravity meter'mass 3 and it is for thispurpose that the present invention is concerned. The relative positionof the gravity meter and its support is observed by a viewer or hairline telescope H which is focused on a fiduciary mark or scale on thesuspended mass.

' In operation a series of locations or stations are selected and onelocation used as the base point for all readings. The instrument isbrought into position at thisv base station and the position of the mass3 observed through the telescope II. The potential of the cylindricalelement 5 is raised or lowered, depending upon the position of the mass,until the fiduciary mark on the mass is brought into the hair lines ofthe telescope. The potential necessary to bring the system to a balancepoint is recorded and used as the zeroor base point for the otherreadings.

After the meter has been balanced at the base station it is moved to theother predetermined locations and the process is repeated, e. g.balancing the system with respect to the fiduciary mark of the suspendedmass by varying the potential of the cylinder I so that the resultingdisplacement of the cylinder and the mass compensates for the change ingravitational force and brings the mass back-to its zero position. Thedifference in the potential of the cylinder between each station iscompared with the zero reading at the base station and the correspondingcomputed.

As the difference in potential is directly proportional to the forcerequired to return the mass to or keep the mass in its original or zeroposition, the calculated forces are an exact measure of the changes ingravitational forces between the corresponding stations and are anindication of the relative magnitude of. the gravitational force at eachlocation.

I claim:

.1. An apparatus for geophysical prospecting that comprises a support, amass resiliently suspended fromsaid support, means to note therelativepositions-of said mass and support, a conductive member fixed tosaid mass, stationary conductive members on opposite sides of saidfirst-mentioned conductive member, means for applying a. potentialbetween said stationary conductive members and means for varying the 1potential between said first-mentioned conductive member and the twostationary conductive members, whereby an electrostatic force is exertedon said first-mentioned conductive memher and through it upon said mass.

2. An apparatus for geophysical prospecting' that comprises a support, amass resiliently suspended from said support, means to note the relativepositions of said mass and support, a conductive member suspended fromsaid mass, stationary conductive members positioned on opposite sides.of and concentric with said firstmentioned conductive member, means forapplying a'constant potential between said stationary conductivemembers, means for varying the potential on said first-mentionedconductive member and means for noting the potential of saidfirst-mentioned conductive member to determine electrostatic fieldbetween said stationary members, means for varying the electricalpotential 01' the first mentioned conductive member so as to introducevarious amounts of asymmetry in the electrostatic field and therebycause a force to act on said first mentioned conductive memher andthrough it upon said mass, and means for noting the potential of saidfirst mentioned.

conductive member.

DAYTON H. CLEWHL.

